Why is adhd considered as a disorder? is it not more of a personality trait?

No: No, it isn't a personality trait, it is a bonafide disorder that can really be poblematic in a persons life. However, it can be treated.

ADHD: It is a disorder with signs & symptoms as classified in the dsm manual of mental illnesses.

Disorder for sure!: Brain studies show differences between persons w/ & w/o adhd, particularly in the left prefrontal cortex. Nimh demonstrated that the brains of kids w/ adhd are 3-4% smaller than those w/o adhd. There is also evidence of differences in neurotransmitters such as norepinephrine & Dopamine (altho the differences are complicated). Adhd causes problems w/ learning, school performance & behavior.